Magnetic reader sensor with shield-to-shield spacing improvement and better free layer-to-shield spacing control

ABSTRACT

An MTJ or MR read sensor is formed by depositing a stack in a reverse order with a free layer (FL) deposited on a lower shield, followed by a tunneling barrier layer (for an MTJ) or a conducting spacer layer (for an MR) and, finally, an antiferromagnetically coupled pinning structure and an upper shield. This reverse order permits a series of etching processes to be accurately performed on the lower shield and the stack together with the formation of biasing layers that are coupled to the lower shield and the stack, without adversely affecting the stability of the pinning structure. Further, the distance between the FL and the shield is accurately determined and repeatable even down to the sub-nm regime. An upper shield can then be formed and also coupled to the biasing layers.

This is a Divisional Application of U.S. patent application Ser. No.15/798,682 filed on Oct. 31, 2017, which is herein incorporated byreference in its entirety and assigned to a common assignee

This Application is related to Ser. No. 15/345,813 Filing Date: Nov. 9,2016, assigned to the same assignee.

BACKGROUND 1. Technical Field

This disclosure relates generally to hard disk drives and particularlyto methods for improving the resolution of MR sensors used to readmagnetic recording media.

2. Description of the Related Art

Current HDD (Hard Disk Drive) designs necessitate the improvement of MRsensor resolution to meet the requirements of increasingly improvingareal density capability of magnetic recording media. One direct methodof providing this improvement is to reduce shield-to-shield spacing ofthe sensor. This approach to improving resolution can be found in theprior arts such as US Published Patent Application 2016/0035378 (Quan etal.), Ser. No. 15/345,813, Filing Date: Nov. 9, 2016, U.S. Pat. No.7,839,605 (Parker) and U.S. Pat. No. 9,355,700 (Sun et al) and U.S.Patent Application 2005/0243475 (Heim).

Some of these prior arts require that pin and antiferromagnetic (AFM)layers be recessed away from the ABS so that the shield to shieldspacing becomes smaller. These approaches involve quite complicatedadditional photo-lithography, etching and refill steps. In addition,alignment between a recessed pin layer and the sensor back edge is alsocritical in determining the sensor performance and reliability.

Reducing the spacing between the free layer (FL) and the closest shieldis another approach to improve resolution. This spacing is normallybetween the FL and the top shield, and the spacing is determined byprocess steps such as the sensor cross-track etching, refill, andtreatments on top of the sensors. This approach results in largedevice-to-device and wafer-to-wafer variations in spacer thickness. As aresult, sensor resolution is not consistent among wafers with similarthin FL cap designs.

This disclosure presents designs that can control the FL-to-shieldspacing quite well, even down to a sub-nm regime, and it simplifies theprocess steps that characterize much of the prior art cited above.

SUMMARY

A first object of the present disclosure is to improve the resolution ofa MR sensor by providing better control and uniformity of the spacingbetween the magnetic free layer (FL) and the closest shield.

A second object of the present disclosure is to meet the first objectwhile also providing better shielding than is currently found in theprior art.

A third object of the present disclosure is to satisfy the above objectswhile requiring simplified process steps in recessing pinned andantiferromagnetic (AFM) layers.

A fourth object of the present disclosure is to provide a device withimproved stability of antiferromagnetically coupled pinning layers.

A sixth object of the present disclosure is to provide a device havingimproved stability of soft biasing layers.

A seventh object of this disclosure is to provide a method of reducingdimensional variations on a device-by-device basis resulting frometching processes.

An eighth object of this disclosure is to provide a more efficient andeffective process flow by virtue of the minimization of process steps.

The above objects will be met by a design and fabrication process for anMR reader (read sensor) that is based on a reversed film stack shown inFIG. 1(a). Unlike the usual reader film stack, the FL is deposited priorto the MgO tunnel barrier layer and pin stack.

Referring to FIG. 1(a), there is shown schematically the followingsequentially layered sensor design. On top of bottom shield S1, 10,which is typically NiFe, 1-2 microns thick in the z-direction, 10-30microns wide in the x-direction and 5-20 microns tall in they-direction. On the shield is first deposited a bottom spacer layer 20.This is followed by deposition of the free layer (FL) 30, then thetunnel barrier layer 40 (or a metallic spacer if we have a GMR sensorinstead of a MTJ sensor) which is typically MgO, and then a pin layerstack 50 on top, typically two magnetic layers (AP1 51 and AP2 53)aligned with anti-parallel coupling between each other by means of a Rulayer 52 inserted in between. In this way, the spacing between the FL 30and the closest shield, which is now S1 10, is defined by the layerthicknesses of the intervening layers to produce a film deposition whichis normally uniform and repeatable, even down to the sub-nm regime.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-1d schematically show the process steps in the formation of thepresent MR reader with controlled FL-to-shield spacing.

FIG. 2a-2c schematically describes shielding schemes of differentreaders in cross-sectional views.

FIG. 3a-3b schematically describes further benefits of the presentdesign.

FIG. 4 is a schematic illustration showing in greater detail how biasinglayers are made for the present design.

FIG. 5a-5c schematically illustrate further design variations of theprevious designs.

FIG. 6a-6c are schematic illustrations showing the formation ofdifferent types of biasing layers involving combinations of hard andsoft bias.

FIG. 7a-7b are schematic illustrations showing the effects of differentetching processes on producing variations in biasing structures.

DETAILED DESCRIPTION

We continue the description of the fabrication process as begun abovewith the description of FIG. 1(a). After the film stack is depositedaccording to FIG. 1(a), we do a cross-track (x-directional) etching ofthe deposited stack first and refill the removed regions with biasinglayers 60 abutting the patterned sides 200 of the stack as isschematically shown in FIG. 1(b). The biasing layer is typically a softmagnetic layer such as NiFe with or without CoFe laminated in, or a hardmagnetic layer as mentioned in FIG. 4-6. An insulating layer 65 isdeposited on biasing layer 60. Layer 65 will prevent subsequent magneticlayers from having direct contact with layer 60 and it is formed of anon-magnetic metal such as Ta, Ru, Cu (which will be shown later inFIGS. 5 and 6, or a non-conducting oxide such as AlOx, SiO₂, or MgO,which will be shown in FIG. 4, to prevent shorting). Note that in thefollowing descriptions we will use soft magnetic material (lowcoercivity) as the biasing layers, but we will also have devicevariations where hard bias (high coercivity) layers are used.

Referring to FIG. 1(c), there is shown the sensor of FIG. 1(b) in a side(y-z plane) cross-sectional view through the center of the fabrication,showing the result of a stripe-height (SH) (y-direction) etching todefine the back edge 300 of the sensor. After the SH direction etching,we refill the back edge with an insulating layer 70 first and then ananti-ferromagnetic (AFM) coupling layer 80, which is typically IrMn orPtMn. It is necessary that after the refill, the sensor top from AP2 toback edge AFM layer top should be quite flat and smooth (co-planar).

After photoresist is removed, we deposit more films, typically anothertwo magnetic layers (AP22 90 and AP3 94) with anti-parallel couplingbetween them by means of the Ru layer 92 sandwiched between them. Then,after an AFM anneal, AP22 90 is coupled to the AFM layer 80 at the backedge of the stack, and through AP22 90 and AP3 94, pinning is alsopropagated to AP2 53 and AP1 51. Then we deposit the top spacer 100 andtop shield (S2) 110. The top shield is typically 40-80 nm thick NiFewith or without CoFe laminated in. The cross-section (y-z plane) viewand ABS (x-z plane) view schematics are shown in FIGS. 1(c) and (d)respectively.

FIG. 2(a)-2(c) schematically shows the shielding schemes of three MRreaders with different designs, from the cross-sectional y-z plane view.The small central rectangle 30 schematically represents the FL, and thetwo rectangles 110 and 10 above and below 30 are the top and bottomshields. FIG. 2(a) is a conventional prior art reader. FIG. 2(b) showsthe high-resolution designs of prior art Refs. US 2016/0035378, whereshield-to-shield spacing is effectively reduced only around the sensorregime. FIG. 2(c) is the design of the current invention, with shield toshield spacing improved both around the sensor regime and at the backedge.

Referring next to FIG. 3(a)-3(b), there is shown two other benefits fromthe present approach. In the cross-track (x-direction) etching shown inFIG. 3(a), we can etch slightly deeper (in the z-direction) 101 intobottom shield S1, 10, to improve the micro track sharpness (signalprofile across the track). However, this is not a good option with aconventional film stack, since deeper etching in a conventional stackalso damages pin layers so that pin robustness becomes worse.

In the stripe height definition etching (y-direction), we can also etchmore deeply into the S1 10 to reduce the presence of FL tails, as shownby the sharply etched back surface 300 in FIG. 3(b). By this process apedestal 12 is formed in S1 and a thicker layer of insulation 710 isadded. In a conventional method, stripe height etching would stop on topof the pin layers, and due to device-to-device variations, many headswould have an un-etched tail left at the back edge. This would be one ofthe reasons for poor sensor performance. Further, if we etch too deeplyinto the pinning layers, pin robustness also becomes worse. The presentmethod does not have these concerns.

The basic structure and main benefits have been described in theprevious paragraph, and here we focus more on the how to form thebiasing layer. One option, shown in FIG. 4, is to couple the soft biaslayer 60, to S1 10. In this design, after cross track direction etchingand insulating layer 65 deposition on top of the bias layer and betweenthe junction side wall of the biasing layer and the sides of the sensor67, there should be another step of removing the insulating layer 68 atthe flat surface on top of S1 while keeping insulator on the junctionside wall 67. This allows the soft biasing layer to be coupled to S1 bycontact, but prevents the junction from shorting to the biasing layerbecause of the intervening insulation 67. Then, after the biasing layerdeposition, the insulating layer 65 is deposited on its top surface toprevent shorting of the biasing layer with the AP layer stack 90, 92 and94.

If we do not want to couple the biasing layer 60 directly to the bottomshield 10 as in FIG. 4, we can now include coupling of the soft biaslayer 60 to the top shield S2 110 in the design and fabrication process.Part of the AP22/coupling layer/AP3/top spacer (90/92/94/100) now abovethe soft bias layer, must be etched away and S2 110 added directly ontop so that it can contact the soft biasing layers 60. This arrangementis shown in FIG. 5(a), where S2 now extends down past the two bufferlayers 200 abutting etched-away sides of the AP22/coupling layer/AP3/topspacer (90/92/94/100) to contact the soft biasing layers at theinterface 250. This approach will require additional photolithographyand etching. Creating and aligning the etching photo-mask is a difficultpart of this design process. In the cross-track x-direction, thephoto-mask must be wider than the width of the sensor stack, yet itcannot be too much wider, because the soft bias layer must have a largeenough area for effective coupling to S2. There are several options foraligning this layer in the vertical direction, and the boundaries withlight and dark lines are shown in top view (x-y plane) schematics ofFIGS. 5(b) and 5(c). Note that the dark lines are the actual edges ofthe sensor and the biasing layers, the light lines show the edges of thepattern to be used for the etching process.

FIG. 5(b) illustrates an option where the vertical (y-direction)boundary of this etching step is below (i.e., closer to the ABS plane)the sensor back edge. The advantage of this option is the strongercoupling in AP22/AP3 90/94. In the present reversed stack design,coupling weakness of AP22/AP3 can be found along the FL back edge, wherethe uneven surface of the back-edge refill, called the “refill stitch”500, is located. With this option, the coupling width of AP22/AP3 overthe stitch is wider, and there will be a reduced chance of pin flip. Thedifficulty is that there is not much room for this boundary to shift inthe vertical (y) direction. If the boundary is too low, the area thatremains to couple the soft bias layers to S2 110 is too small, so thatinstabilities may occur in the soft bias.

The second option is to have the vertical boundary 520 above the sensorback edge 530, as shown in FIG. 5(c). In this case we will have enougharea for effective coupling of the soft bias layers to S2, but thecoupling width of AP22/AP3 is smaller, and we have some areas where AP2is directly coupled to S2 on the side. But the impact of this directlycoupled region to the sensor should be small since it is quite far fromthe sensor. To balance these two effects, we can create the flare-outshape of this layer shown with the lighter dashed lines in FIG. 5(c), sothat the AP22/AP3 coupling width is larger without losing as much softbias to the S2 coupling area.

The present method also allows several ways to construct a hard biasinglayer. FIG. 6(a) illustrates a particularly straightforward way which todo this, which is to deposit a layer 61 with very high coercivity(typically CoPt or FePt) in place of the soft biasing layer of FIG.1(d). This can be done using the same design as in FIG. 1(d). Thisapproach is now shown in FIG. 6(a), where, for simplicity, upper andlower shield layers are not shown and only the bias layers 60 and aninsulating layer 65 are identified by number because all other layersare exactly as shown in FIG. 1(d) with exception of the lower and uppershields. However, it has been shown that when using the present design,the soft biasing layer (typically NiFe) has better on-track performanceas well as cross track sharpness, although the soft biasing layer needsto be pinned by some other layer whose magnetization cannot be easilyre-oriented.

Another way to introduce hard magnetic bias layers is to couple the NiFesoft magnetic bias layer 60 to a CoPt or FePt hard bias layer 66, asshown in FIG. 6(b). Note that the hard bias layer here no longerprovides a biasing field and, therefore, it only needs a highcoercivity. It is not necessary that it has a large enough moment toprovide the field. As in FIG. 6(a), only the biasing layers are numberedfor reference.

Another possibility is to couple the soft biasing layer to an additionalAFM layer 68, as shown in FIG. 6(c). But care must be taken with respectto its thickness and properties. When the pin layer's AFM at the backedge (see 80, in FIG. 1(c)) is being annealed, the biasing layer AFM 68has already been deposited. Therefore the AFM biasing layer should bethin enough to be re-oriented at a later anneal, with the temperaturebelow the blocking temperature of the pin layer, and it also should notbe too thin so as to cause concern over its thermal robustness.

Referring now to FIGS. 7(a) and 7(b) there are shown two views of theresults of doing stripe height etching before the cross-track etching.Using this process option, as shown in 7(a), a soft biasing layer 60 canbe coupled directly to top shield S2 because theAP22/coupling/AP3/spacer layers have been etched away laterally and donot intervene between the top shield 110 and the bias layers 60. Asshown in 7(b), the stripe-height (y) direction has been done first, andthe rear of the device 300 is trimmed. Hard magnetic bias is alsopossible with this process flow. Note also that the combined pinningstack of AP1/Ru/AP2/AP22//Ru/AP3, 51/52/53/90/92/94, (using Ru for thecoupling layer), is to use antiparallel orientations of theferromagnetic layers to cancel the presence of a dipolar field from anet magnetic moment at the position of the FL. If this is not required,other schemes of the AP stack are possible.

As is finally understood by a person skilled in the art, the detaileddescription given above is illustrative of the present disclosure ratherthan limiting of the present disclosure. Revisions and modifications maybe made to methods, materials, structures and dimensions employed informing and providing a shielded MTJ or MR read sensor with improvedresolution resulting from more precise shield-to-free layer spacing,while still forming and providing such a structure in accord with thespirit and scope of the present invention as defined by the appendedclaims.

What is claimed is:
 1. A method of forming an MTJ or MR read sensor,comprising: providing a bottom shield layer; sequentially forming onsaid bottom shield layer a reversed thin-film MTJ or MR sensor stackdeposition, wherein a spacer layer is formed on said bottom shieldlayer, a free layer (FL) is formed on said spacer layer, a barrier layer(for an MTJ) or conducting layer (for an MR) is then formed on the FLand a pinned layer structure is formed thereon; then patterning saidsensor stack deposition in a cross-track (x) direction to removelaterally extending portions of each layer formed sequentially on saidbottom shield layer and to reduce the stack width and producesymmetrically sloping sides; then forming a first layer of insulationover said bottom shield layer and over said symmetrically sloping sides;then forming biasing layers over said layer of insulation, said biasinglayers abutting said insulation layer covering said symmetricallysloping sides; then forming a second layer of insulation over saidbiasing layers; then patterning a back end of said sensor stackdeposition in a stripe-height (y) direction, reducing the height of alllayers thereof and producing a sloping side thereon; then forming aninsulating refill layer covering said sloping side on said back end andextending down said sloping side and forming a horizontal portionextending horizontally rearward; then forming an antiferromagnetic (AFM)coupling layer on said horizontal portion of said insulating refilllayer, said AFM coupling layer abutting said insulating refill layercovering said sloping side; and forming a smooth surface in which a topsurface of said AFM coupling layer and said pinned layer structure areco-planar.
 2. The method of claim 1 wherein said pinned layer structurecomprises an upper ferromagnetic layer AP2, a lower ferromagnetic layerAP1 and a coupling layer inserted between AP2 and AP1 whereby AP2 andAP1 are magnetically coupled in antiparallel directions.
 3. The methodof claim 1 further comprising: forming, on said smooth surface, a pairof magnetic layers comprising a top layer and a bottom layer coupled inan antiparallel configuration by means of a coupling layer formedbetween them; then forming a spacer layer on an upper surface of saidtop layer; and forming an upper shield layer on said spacer layer. 4.The method of claim 3 wherein said biasing layers are layers of softmagnetic material.
 5. The method of claim 3 wherein said biasing layersare layers of hard magnetic material.
 6. The method of claim 3 whereinsaid biasing layers are layers of soft magnetic material on each ofwhich is formed a layer of hard magnetic material.
 7. The method ofclaim 3 wherein said biasing layers are layers of soft magnetic materialon each of which is formed a layer of antiferromagnetic material.
 8. Themethod of claim 7 wherein said layer of antiferromagnetic material mustbe of such material and of such dimensions that it can have itsmagnetization re-oriented subsequent to an anneal of saidantiferromagnetic coupling layer abutting the insulating refill layer.9. The method of claim 3 further comprising: patterning said pair ofmagnetic layers comprising said top layer and said bottom layer coupledin an antiparallel configuration by means of said coupling layer formedbetween them; then forming said spacer layer on said upper surface ofsaid top layer; and forming said upper shield layer on said spacerlayer.
 10. The method of claim 1 further comprising: forming, on saidsmooth surface, a pair of magnetic layers comprising a top layer and abottom layer coupled in an antiparallel configuration by means of acoupling layer formed between them; then forming a spacer layer on anupper surface of said top layer.
 11. The method of claim 10 furthercomprising: patterning said coupled pair of magnetic layers and saidspacer layer above and beneath them to uniformly reduce the width andheight of all said layers so that said layers extend symmetrically inthe x and y directions further than the corresponding extent of said FLand produce a layered rectangular prism having a planar top surface, anABS surface, a rear surface and two parallel side surfaces; then formingan upper shield to partially surround said layered rectangular prismwhereby said upper shield covers the top surface, the rear surface andboth side surfaces of said rectangular prism while leaving said ABSsurface uncovered and wherein said surrounding portion of said uppershield extends downward to contact an upper surface of said biasinglayers thereby coupling to said biasing layers and stabilizing them.